Some current medical devices designed to provide antitachycardia therapy have attempted to discriminate between physiological (e.g. NSR) and pathological cardiac rhythms (e.g., SLVT) using the heart rate sensed by a single ventricular sensor. U.S. Pat. No. 4,875,483 to W. Vollmann et al., entitled "Implantable Cardiac Pacer With Programmable Antitachycardia Mechanisms", which issued on Oct. 24, 1989, discloses such a device. However, techniques which are based solely on ventricular rate parameters are unable to distinguish between physiological and pathological rhythms that have overlapping ventricular rates. An important instance of this is normal sinus rhythm (a physiological rhythm) and slow ventricular tachycardia (a pathological rhythm). The addition of an atrial sensor providing atrial event information may overcome this particular problem, as is proposed in U.S. Pat. No. 4,860,749 to M. H. Lehmann, entitled "Tachycardia Detection for Automatic Implantable Cardioverter/Defibrillator With Atrial and Ventricular Sensing Capability", which issued on Aug. 29, 1989. The device disclosed in U.S. Pat. No. 4,860,749 offers only a partial solution to this problem by classifying the rhythm as VT1:1R when the AV interval exceeds the sinus AV interval, provided the ventricular cycle length exceeds a crossover value. The device described in U.S. Pat. No. 4,860,749 is, however, still not capable of identifying SDVT and, furthermore, fails to deal with ventricular ectopic beats (VEB).
Accordingly, the main object of this invention is to reliably discriminate between normal sinus rhythm (NSR) and slow ventricular tachycardia (SLVT). The term SLVT encompasses both ventricular tachycardia with 1:1 retrograde conduction (VT1:1R) and slow dissociated ventricular tachycardia (SDVT).
It is another object of the invention to provide a classification technique which copes with ventricular ectopic beats (VEB).
Further objects and advantages of the invention will become apparent as the following description proceeds.